1. Field of the Invention
The present invention relates to a color cathode-ray tube (CRT) of a dot-in-line type. More specifically, the present invention pertains to a color CRT in which electron guns are in an in-line array and apertures of a shadow mask are circular and particularly to an array of the apertures.
2. Description of the Prior Art
A color CRT of a dot-in-line type includes a set of three electron guns in an in-line array for emitting three electron beams for three different colors. Ahead of the electron guns, a shadow mask with a curved surface is disposed in a direction intersecting with the electron beams and, further ahead of the shadow mask, a fluorescent screen having a curved surface similar to the curved surface of the shadow mask is provided in parallel with the shadow mask. The shadow mask has a large number of small circular apertures, in a dotted manner, instead of elliptical or slitted apertures.
One of the problems in the landing characteristics of the electron beams in such a conventional dot-in-line type color CRT is, as described in the official gazettes of Japanese Patent Publication Nos. 21214/1975 and 19909/1965, that the pattern of arrival points of the electron beams on the fluorescent screen does not become one having a close packed structure.
FIG. 1 is a schematic illustration for explaining a relation between a set of electron beams in an in-line array and an array of apertures of a shadow mask. In FIG. 1, the in-line array of electron beams is provided parallel to the horizontal direction of a fluorescent screen 1, electron beams for blue, green and red colors being shown on an enlarged scale as BB, BG and BR, respectively. As shown in FIG. 1, it is assumed that an X axis parallel to the in-line array passes through the center of the fluorescent screen and a Y axis vertical to the X axis also passes through the center. In an inserted circle A in FIG. 1, a part of an array of apertures of a shadow mask 2 set near the fluorescent screen 1 is shown on an enlarged scale, with a broken line B as one of lines connecting the nearest apertures out of the apertures 2a being parallel with the Y axis. The fluorescent screen 1 is generally formed on the inner surface of a glass panel. Since this glass panel finally constitutes a vacuum vessel for serving as a CRT, the panel is formed to have a spherical surface to prevent an explosive break thereof and the shadow mask 2 is also formed to have a spherical surface corresponding to the fluorescent screen 1.
FIG. 2 is a schematic illustration showing an inclination of a trio of electron beams in each corner portion of the fluorescent screen 1. In FIG. 2, the characteristics of a pattern of arrival points of a trio of electron beams BB, BG and BR in the respective portions of the fluorescent screen 1 after passing through an aperture 2a of the shadow mask 2 are illustrated in an exaggerated manner on an enlarged scale. More specifically, since the fluorescent screen 1 has a spherical surface, the arrival points of the trio of beams BB, BF and BR, for example in a corner portion C of the first quadrant, are arrayed downward to the right. FIG. 3 is a detailed illustration, on an even more enlarged scale, showing the arrival points of the electron beams in the corner portion C shown in FIG. 2. In FIG. 3, the arrival points of a trio of electron beams through an aperture 2-1 (not shown) of the shadow mask 2 are shown as B.sub.1, G.sub.1 and R.sub.1, and similarly the arrival points of trios of electron beams through an adjacent aperture 2-2 (not shown) on the right of the aperture 2-1 and an adjacent aperture 2-3 (not shown) on the downward right of the aperture 2-2 are shown as B.sub.2, G.sub.2 and R.sub.2, and B.sub.3, G.sub.3 and R.sub.3, respectively. As seen in FIG. 3, there is a problem that a broken line D connecting the points B.sub.1, G.sub.1 and R.sub.1 and a broken line E connecting the points B.sub.2, G.sub.2 and R.sub.2 are not continuous between the points R.sub.1 and B.sub.2, causing a step therebetween. In addition, since a broken line F connecting a trio of arrival points B.sub.3, G.sub.3 and R.sub.3 of electron beams through the aperture 2-3 is also inclined as shown in FIG. 3, a distance between the points B.sub.3 and R.sub.1 becomes extraordinarily short, which constitutes one of the particularly disadvantageous aspects of the landing characteristics in the dot-in-line type of color CRTs.
FIG. 4 is a schematic illustration showing an ideal case corresponding to FIG. 3. The arrival points of electron beams in FIG. 4 form a close packed structure, in which regular triangles respectively formed by the nearest three arrival points are laid most densely. In such an ideal case, the fluorescent screen is utilized most effectively. In other words, the space utilization factor of the screen is at its optimum.
FIG. 5 is a schematic illustration representing a distribution of apertures in a shadow mask utilized in the above described Japanese Patent Publication No. 19909/1975 with a view to avoiding such distortion as shown in FIG. 3. Referring to FIG. 5, the X axis is parallel to the in-line direction, passing through the center of the shadow mask and the Y axis passing through the center is vertical to the X axis. As seen in FIG. 5, rows of apertures of the shadow mask in the direction of the X axis are curved in the shape of a barrel, while columns of apertures in the direction of the Y axis are curved in the shape of a pincushion. Such array of apertures makes it possible to correct such distortion as shown in FIG. 3 to take a closer approach to the ideal state as shown in FIG. 4. However, the rows and columns of such array of apertures are not parallel to the X axis or to the Y axis and, as a result, the manufacturing of such a mask is complicated and expensive.
On the other hand, some other arrays of apertures are proposed for the purpose of solving the above described problem. However, they are improved arrays only in one dimension (only in one direction) and none of them can solve the above described problem satisfactorily.